Optical properties of semiconductor nanocrystals are controlled by their respective diameter when reducing scales to about 10 nm. This effect is called “quantum confinement” and is one of the important properties of nanoscaled semiconductor materials for quantum optics and photonics. In this lecture I will outline the basic principles of the quantum confinement effect. Special emphasis will be on applications in life sciences especially when applying single molecule detection schemes. Closely related to this lecture is lecture S5, which will concentrate on the related physico- chemical aspects.